UBC Theses and Dissertations
Advances in modelling and material characterization of hole spin qubits in Ge Ciocoiu, Antonia
Quantum computers have the ability to perform calculations that are currently intractable on the best supercomputers. While the creation of a universal quantum computer is far removed, an exciting near-term application of quantum computing lies in quantum simulation. Realization of these simulators would allow for computing things like many-particle interactions in materials, leading to scientific advancements. Spin qubits using holes in quantum dots are a promising option towards implementing these near-term quantum simulators, since they have been shown to have long coherence times and are compatible with existing silicon manufacturing technology, making them suitable for scaling. State-of-the-art hole spin qubit systems include the development of two and four-qubit processors. Despite their potential, little work has been made towards enabling scalability of hole-spin qubit platforms. Current simulations employ computational approaches that rely on simplified models and do not take into account device design. In this thesis we develop a simulation framework for hole spin qubits in Ge using realistic gate patterns and examine the effect of this model on key properties of these qubits. We determine that significant differences emerge between the simplified and realistic models, and we quantify these differences. This work defines a minimal model that can be used to predict and optimize spin qubit performance. Germanium is often used for spin qubit platforms because it forms a good ohmic contact with most metals. In doped Ge quantum wells, holes have high mobility and low effective mass, facilitating the confinement of spins. However, highly doped materials contain impurities that degrade the performance of quantum devices at low temperature. Although unexplored, there is interest in using undoped and unstrained Ge epilayers in quantum devices, since they have a higher thermal budget than quantum wells and are an industrial material, allowing for more freedom in fabrication and scaling. In this thesis, we characterize a Ge epilayer substrate by designing and fabricating field-effect transistors and Hall bars to determine the electrical transport properties, carrier concentration and mobility at 4K. We determine that the device displays current-voltage characteristics typical of a transistor. Together, these results advance knowledge on hole spin qubits in Ge.
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